This invention relates to controls for a variable displacement hydrostatic transmission, hereinafter designated "HST".
HSTs are known to be useful for propelling many types of vehicles. They consist of at least one pump and may include a plurality of motors. However, a persistent problem is that two distinct modes of operation are often desired by the operator. In the first mode, the operator desires to utilize the variable speed capability of the hydrostatic transmission, at a relatively constant engine speed, to perform a work function. In this mode, the relatively low vehicle speed is a function of the engine speed, a constant, and the variable displacements of the components of the hydrostatic transmission. In the second mode, the operator desires to utilize the HST for travel by propelling the vehicle at a speed proportional to engine speed. Typically, much higher speeds are desired in the travel mode then the work mode. Automotive-style control, where engine speed and vehicle speed can be proportionally affected by an accelerator foot pedal, is often desired in the travel mode.
To provide a travel mode and work mode, vehicle designers have sometimes been forced to use separate transmissions for each mode. Such redundancy is both costly and inefficient. The street sweeper of U.S. Pat. No. 4,553,626 discloses an attempt to use the same HST for both modes by linking an inching valve with a selector lever, throttle, and governor. This results in an accelerator-controlled travel mode and a governor-controlled work mode. U.S. Pat. No. 4,554,992 discloses a four wheel street sweeper which uses a HST controlled by an inching valve and an automatic metering speed control valve to give high and low speed operation. U.S. Pat. No. 5,056,615 discloses a speed control which results in separate modes for travel and an auxiliary drive for vehicles such as street sweepers.